1. Field of the Invention
The present invention relates to a method and apparatus for focus control of an optical disk drive that provides stable focus control performance in the case of rapid pickup movement.
2. Description of the Related Art
When data recorded in a particular area on an optical disk is requested, rough and fine search operations are performed in sequence. The rough search operation is to move the optical pickup near to the target track by applying a large drive voltage to the sled motor moving the optical pickup. If the optical pickup approaches the target position by the rough search, the fine search operation begins. The optical spot is precisely positioned on the target track by the fine search operation.
In the case where the optical pickup moves rapidly for accessing the target track, the objective lens installed in the optical pickup is likely to vibrate due to rapid movement and as a result the focus error tends to increase abruptly during rough search operations as shown in (c) of FIG. 1. Generally, the focus servo loop is designed so that the axial vibration of the disk at the disk rotation frequency (for example 100 Hz) may be sufficiently attenuated by increasing the loop gain at the frequency, as shown in FIG. 2.
However, such a design scheme increases phase lag at the frequency range 1xcx9c2 KHz as shown in FIG. 3. Since the vibration of the objective lens belongs to the frequency range, the overall focus servo loop may be unstable during rough search operations due to the increased phase lag. At the worst, the overall focus servo loop diverges to be interrupted. To prevent the problem, the speed of the optical pickup should be limited during rough search operations, which is the major obstacle to implementation of high-speed access systems.
It is an object of the present invention to provide a method for focus control of an optical disk drive that allows for stable focus control performance in the case of track search operations.
The apparatus for focus control of an optical disk drive in accordance with the present invention comprises a driving unit to rotate the optical disk, a signal detecting unit to detect signals recorded on the optical disk, a focus servo unit to focus the signal detecting unit on the rotating optical disk in accordance with set operating characteristics, and a control unit to change the operating characteristics of the focus servo unit depending upon whether the signal detecting unit is reproducing recorded signals or traversing tracks.
The method for focus control of an optical disk drive in accordance with the present invention comprises the steps of rotating the optical disk, compensating the axial vibration of the rotating optical disk with focus servo loop characteristics initially set, and modifying the focus servo loop characteristics and performing a rough search operation to rapidly move the optical pickup if access to a target position is requested.
In addition, this focus control method and apparatus changes focus servo loop characteristics for a feature of each section of radial path of a disk, thereby also resolving a problem that a moving speed of an optical pickup is restrained due to vibration of an objective lens, which results in improvement of access speed and stable rapid search.